The invention relates to packet-switched communications in mobile networks.
Mobile communications systems, such as cellular or personal communications services (PCS) systems, are made up of a plurality of cells. Each cell provides a radio communications center in which a mobile unit establishes a call with another mobile unit or a wireline unit connected to a public switched telephone network (PSTN). Each cell includes a radio base station, with each base station connected to a mobile switching center that controls processing of calls between or among mobile units or mobile units and PSTN units.
From the original advanced mobile phone system (AMPS) standard, additional wireless protocols have been developed and implemented. One such protocol is the time-division multiple access (TDMA) TIA/EIA-136 protocol from the Telecommunications Industry Association (TIA). With TIA/EIA-136 TDMA, each channel carries a frame that is divided into six time slots to support up to three or six mobile units per channel. Other TDMA-based systems include Global System for Mobile (GSM) communications systems, which use a TDMA frame divided into eight time slots (or burst periods).
Traditional speech-oriented wireless systems utilize circuit-switched connection paths in which a line is occupied for the duration of the connection between a mobile unit and the mobile switching center. Such a connection is optimum for communications that are relatively continuous, such as speech. However, data networks such as local area networks (LANs), wide area networks (WANs), and the Internet use packet-switched connections, in which communication between nodes on a communications link is performed with data packets. Each node occupies the communications link only for as long as the node needs to send or receive data packets. With the rapid increase in the number of cellular subscribers in conjunction with the rising popularity of communications over data networks, a packet-switched wireless data connection that provides access to the data networks, electronic mail, web sites, and other features has become increasingly desirable.
A wireless connection protocol that has been proposed to provide more efficient connections between a mobile unit and a packet-switched data network such as an Internet Protocol (IP) network is the General Packet Radio Service (GPRS) protocol, with versions complementing existing GSM systems and TIA/EIA-136 systems. In a GPRS communications system, various entities are present. A serving GPRS support node (SGSN) controls communications between mobile units and a packet-based data network. The SGSN is typically connected to a gateway GPRS support node (GGSN), which provides the interface to the packet-switched data network. The SGSN is connected to base station systems (BSS) over respective Gb interfaces, which provide for the exchange of control signaling and user data.
The Gb interface link layer may be based on the Frame Relay protocol, as described in TS 101 299, entitled “Digital Cellular Telecommunications System (Phase 2+); General Packet Radio Service (GPRS); Base Station System (BSS)-Serving GPRS Support Node (SGSN) Interface; Network Service,” GSM 08.16 Version 6.1.0, Release 1997 (hereinafter the “GSM 08.16 Standard”). In a Frame Relay network, permanent virtual connections (PVCs) are established between the SGSN and each base station system. A PVC is a predetermined logical path through the network between two points, in this case the SGSN and a base station system. Each PVC is associated with a data link connection identifier (DLCI), which is the identification of the PVC used by the network to find the right path and destination for a communicated frame of data. A further description of the Gb interface is provided in Draft ETSI EN 301 344, entitled “Digital Cellular Telecommunications System (Phase 2+); General Packet Radio Service (GPRS); Service Description; Stage 2,” GSM 03.60 Version 7.1.0, Release 1998 (hereinafter the “GSM 03.60 Standard”).
Communications according to the Frame Relay protocol are connection-oriented, and differ from communications over connectionless, packet-switched networks such as IP networks. The Frame Relay protocol is relatively tightly coupled to the underlying physical layer, such as a T1 or T3 layer. This limits the flexibility in how the Gb interface can be implemented. A need thus exists for a more robust and flexible interface between base station systems and a system controller such as the SGSN in a mobile communications system.